FIG. 1 illustrates a twin spool, high bypass gas turbine aircraft engine. The first spool includes a shaft 3 which carries a fan 6, a booster compressor 9, and a low-pressure turbine 12. A second spool includes a shaft 15 which carries a high-pressure compressor 18 and a high-pressure turbine 21. In operation, an incoming airstream 24 is compressed by booster 9, further compressed by high-pressure compressor 18, and delivered to a combustor 27. Therein, fuel is injected, the mixture burns, expands, and exhausts in sequence through the high-pressure turbine 21 and the low-pressure turbine 12, providing energy to rotate the turbines, the compressors, and the fan 6. The fan generates a propulsive airstream 30.
The clearance, represented by dimension 33, between the high-pressure turbine 21 and a shroud 36 which surrounds it, must be maintained as small as possible in order to prevent leakage of air through the clearance 33. Leaking air imparts little or no momentum to the turbine, and thus represents a loss in energy. One possible solution to the leakage problem may be thought to lie in the expedient of manufacturing the engine such that the clearance 33 is a small dimension, such as 1/1000th of an inch. However, this approach is not feasible, as FIG. 2 will illustrate. In that figure, the turbine blades and the shroud are shown in two states, namely, their cold, unexpanded state, labeled by numerals 40 and 42, and their hot, expanded state, drawn in phantom and indicated by numerals 44 and 46.
The expansion of the turbine rotor can be viewed as resulting from the combined effects of three factors: (1) centrifugal expansion of the turbine rotor disc occurring from ground idle to takeoff, which is indicated by numeral 123 in FIG. 1, and which can amount to an increase in radius of the turbine blades (dimension 49A in FIG. 2) of about 0.020 inches; (2) thermal expansion of turbine rotor disc 123 in FIG. 1, which is approximately equal to the 0.065 inch centrifugal expansion; and (3) thermal expansion of the blades themselves, which increases the dimension 49A in FIG. 2 by about 0.005 inch.
At about the same time that the tip radius 49A in FIG. 2 is changing, the hot gas stream passing through the turbine blades causes the shroud 44 to expand to phantom position 46. In particular, during an acceleration from ground idle to a speed of 14,500 rpm in the high pressure turbine 21 in FIG. 1, the events just described occur in generally the following sequence: (1) centrifugal expansion of the rotor disc, which is immediate, followed by (2) blade thermal expansion, followed by (3) shroud thermal expansion, and, finally, (4) rotor disc thermal expansion.
Although this sequence is oversimplified, since the actual cooperation of the four factors is more complex than just described, the following principle is clear. Given the dimensional changes assumed, the clearance 33, when the components are non-rotating, must exceed 0.025 inches, because the centrifugal expansion of the disc (0.020 inch), together with the thermal expansion of the blades 40 (0.005 inch), will consume this clearance, before the thermal expansion of the shroud 42 will move the shroud out of the way. However, this clearance of 0.025 inches allows leakage losses at the blade tips which are preferably avoided.
Further, the thermal expansion of the shroud 42 from the solid position shown to the phantom position 46 is about the same as the thermal expansion of the rotor disc, which is about 0.020 inches, as stated above. However, as also stated, the shroud thermal expansion precedes disc thermal expansion by 10 to 30 minutes, depending on rotor rpm. Therefore, during this period, an unwanted clearance of up to 0.020 inches can exist.